1. Technical Field
This invention is directed toward a new whiteboard capture camera design. More specifically, this invention is directed toward a camera system and device for providing uniform resolution images of a whiteboard.
2. Background Art
Many meeting scenarios use a whiteboard extensively for brainstorming sessions, lectures, project planning meetings, patent disclosures, and so on. Note-taking and copying what is written on the board often interferes with many participants' active contribution and involvement during these meetings. As a result, some efforts have been undertaken to capture whiteboard content in some automated fashion.
Several technologies have been developed to capture whiteboard content automatically. More recent technologies attempt to capture the whiteboard content in digital form from the start, especially in image form. National Television System Committee (NTSC)-resolution video cameras are often used because of their low cost. Since these cameras usually do not have enough resolution to clearly capture what is written on a typical conference room size whiteboard, several video frames must be stitched together to create a single whiteboard image.
Images captured with a high resolution digital video camera provide contextual information such as who was writing and which topic was being discussed. Traditional whiteboard cameras have a lens parallel to the image plane with a center of projection that intersects the camera sensor's central ray. The lens axis is carefully aligned “normal” to the image sensor. That is, the lens axis is perpendicular to the image sensor. Such an alignment of lens and image sensor causes the camera to be focused on a subject plane that is parallel to the image sensor.
One problem with capturing whiteboard images using this traditional camera configuration is, however, that this lens configuration results in dramatically varying resolution for portions of the whiteboard. For instance, if the camera is pointed at the top portion of the whiteboard, the lower portion of the whiteboard will typically require a much higher image resolution in order for content written on the bottom portion of the whiteboard to be readable. The resolution can differ dramatically from the top and bottom of the whiteboard, by as much as a factor of 3 times for a typical setup. Accordingly, most whiteboard camera implementations require high-resolution image sensor cameras to ensure that the entire whiteboard image has adequate image resolution to provide legible images of the entire whiteboard.
A second problem with capturing whiteboard images using this traditional camera configuration is that the focus plane is not parallel to the whiteboard plane. To overcome this second problem, a small aperture could be used, but that reduces light on the sensor and requires higher image gain, which induces image noise.
View cameras, which have been known for years, are expressly designed to let the lens and film alignment to be adjusted outside the “normal” conditions where the lens axis is aligned normal to the film. The view camera is based on a rule that is known as the Scheimpflug Principle. In 1904, Theodor Scheimpflug, described a device for correcting for the geometric distortion in aerial photographs taken when the camera lens was not pointing straight down. By applying this correction he was able to produce accurate maps. Scheimpflug's invention was based on a similar but earlier camera-like apparatus patented by Jules Carpentier in 1901. Carpentier experimented in coordinating the tilting movements of the subject plane and film plane (equivalent to the sensor plane in a digital camera) in order to keep the image in focus. Scheimpflug built on Carpentier's observations by showing that in order to achieve perfect focus, it was necessary that the subject plane, the film plane and the lens plane intersect along one common line. The lens plane is defined as the plane surface passing through the center of the lens, perpendicular to the lens axis.
The fact that this plane passes through the Scheimpflug Line is not enough to ensure focus, however. Scheimpflug's principle alone is not always very useful. There are an infinite number of ways to adjust the focus of a camera, each way obeying the Scheimpflug Principle but still not achieving the desired focus. However, there is a second law that applies to view cameras.
The second law describes three planes that must intersect along a single line. One of these planes is the aforementioned subject plane. The second plane is one through the center of the lens but parallel to the film, called the parallel-to-film lens plane, or PTF plane. The third plane is one parallel to the usual lens plane but one focal length in front of it. The second law, standing alone, defines the amount of lens tilt necessary for any given situation. The distance from the lens to where the PTF plane and the subject plane intersect is the only factor determining how much lens tilt is needed. This second law as described above has been called “the hinge rule”. The hinge rule gives the hinge line through which the plane of sharp focus will pass. For sharp focus, the hinge line must be on the subject plane. The position of the camera back, through the Scheimpflug Principle, then sets the angle at which the plane of sharp focus will pass through the hinge line. Moving the camera back and forth causes the plane upon which the camera is focused to rotate about the hinge line.
The depth of field of the view camera is also closely related to the hinge rule. As stated above, the hinge rule states that as the lens-to-film distance is adjusted through simple focusing motions of the camera back, the subject plane rotates as though it were hinged at a specific location in space. That location is determined by the amount of lens tilt (relative to the camera back) and the direction of that tilt. The line about which the subject plane pivots is the hinge line. The distance from the lens to the hinge line is controlled by the amount of tilt and the focal length of the lens. The limits of the depth of field are established by just three factors: the hyperfocal distance (H) for the focal length and aperture in use, the position of the plane of sharpest focus, and by the lens-to-hinge line distance (called J). The hyperfocal distance is the distance at which one would focus a non-view camera in order to put the far limit of depth of field at infinity. H usually corresponds to about 1500 times the physical diameter of the lens aperture. At the hinge line the depth of field is zero. One hyperfocal distance in front of the camera, the depth of field, measured vertically (parallel to the camera back) is (approximately) equal to that distance J. At other distances depth of field scales with distance. At a distance H/2, the depth of field is J/2 and so on. A minor correction factor is necessary under close-up conditions.
In a view camera, depth of field is influenced directly by the distance J, which is in turn determined by the amount of lens tilt used. More lens tilt results in less depth of field. Tilting the lens can improve overall sharpness by orienting the plane of sharpest focus to coincide with the subject. But tilting the lens also reduces the amount of depth of field on either side of that plane.